Termination circuit for on-die termination

ABSTRACT

In a semiconductor device having a terminal connected to an internal portion, a termination circuit for providing on-die termination for the terminal of the device. The termination circuit comprises a plurality of transistors, including at least one NMOS transistor and at least one PMOS transistor, connected between the terminal and a power supply; and control circuitry for driving a gate of each of NMOS transistor with a corresponding NMOS gate voltage and for driving a gate of each PMOS transistor with a corresponding PMOS gate voltage, the control circuitry being configured to control the NMOS and PMOS gate voltages so as to place the transistors in an ohmic region of operation when on-die termination is enabled. The power supply supplies a voltage that is less than each said NMOS gate voltage and greater than each said PMOS gate voltage.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation under 35 U.S.C. §120 of U.S.patent application Ser. No. 12/685,365 to Peter B. Gillingham, filedJan. 11, 2010 now U.S. Pat. No. 8,063,658, and claims the benefit under35 USC §119(e) of U.S. Provisional Patent Application Ser. No.61/151,886 to Peter B. Gillingham, filed Feb. 12, 2009. The aforesaidapplications are hereby incorporated by reference herein.

BACKGROUND

When a signal travels along a path that has an impedance discontinuity(or “mismatch”), the signal is partly reflected. The reflected signalinterferes with the original signal and this can result in a loss ofsignal integrity and an incorrect signal level being detected by areceiver. To mitigate the onset of signal reflection, it is beneficialto place circuitry with the equivalent amount of impedance at the pointof discontinuity. This is referred to as “termination”. For example,resistors can be placed on computer motherboards to terminate high speedbuses.

Although termination resistors reduce reflections at an extremity of thesignal path, they are unable to prevent reflections resulting from stublines that connect to other semiconductor chips at various points alongthe path. This situation can arise, for example, when multiple memorymodules are connected along a memory bus. A signal propagating from amemory controller along the memory bus encounters an impedancediscontinuity at each stub line leading to a particular memory module.The signal that propagates along the stub line leading to the particularmemory module will be reflected back onto the memory bus, therebyintroducing unwanted noise into the signal.

Accordingly, it is useful to provide each semiconductor chip with itsown termination circuitry. Providing this termination circuitry on thesame semiconductor chip that includes a bus transmitter an/or receiveris known as on-die termination (ODT). On-die termination can reduce thenumber of resistor elements and complex wirings on the motherboard.Thus, in addition to improved signal integrity, which allows componentsto be operated at higher frequencies, on-die termination enables asimpler and more cost effective system design.

However, conventional on-die termination techniques have tended to bepower hungry and/or inflexible.

SUMMARY

According to a first broad aspect, the present invention seeks toprovide, in a semiconductor device having a terminal connected to aninternal portion, a termination circuit for providing on-die terminationfor a terminal of the semiconductor device. The termination circuitcomprises a plurality of transistors connected between the terminal anda power supply, the plurality of transistors including at least one NMOStransistor and at least one PMOS transistor; and control circuitry fordriving a gate of each of the at least one NMOS transistor with acorresponding NMOS gate voltage and for driving a gate of each of the atleast one PMOS transistor with a corresponding PMOS gate voltage, thecontrol circuitry being configured to control the NMOS and PMOS gatevoltages so as to place the plurality of transistors in an ohmic regionof operation when on-die termination is enabled. The power supplysupplies a voltage that is less than each said NMOS gate voltage andgreater than each said PMOS gate voltage.

According to a second broad aspect, the present invention seeks toprovide a semiconductor device with on-die termination, which comprisesan internal portion; a power supply; a terminal connected to theinternal portion; a plurality of transistors connected between theterminal and the power supply, the plurality of transistors including atleast one NMOS transistor and at least one PMOS transistor; and controlcircuitry for driving a gate of each of the at least one NMOS transistorwith a corresponding NMOS gate voltage and for driving a gate of each ofthe at least one PMOS transistor with a corresponding PMOS gate voltage,the control circuitry being configured to control the NMOS and PMOS gatevoltages so as to place the plurality of transistors in an ohmic regionof operation when on-die termination is enabled. The power supplysupplies a voltage that is less than each said NMOS gate voltage andgreater than each said PMOS gate voltage.

According to a third broad aspect, the present invention seeks toprovide a semiconductor device with on-die termination, which comprisesan internal portion; a power terminal for connection to an off-chippower supply; a data terminal connected to the internal portion; aplurality of transistors connected between the data terminal and thepower terminal, the plurality of transistors including at least one NMOStransistor and at least one PMOS transistor; and control circuitry fordriving a gate of each of the at least one NMOS transistor with acorresponding NMOS gate voltage and for driving a gate of each of the atleast one PMOS transistor with a corresponding PMOS gate voltage, thecontrol circuitry being configured to control the NMOS and PMOS gatevoltages so as to place the plurality of transistors in an ohmic regionof operation when on-die termination is enabled. The power terminalsupplies a voltage that is less than each said NMOS gate voltage andgreater than each said PMOS gate voltage.

According to a fourth broad aspect, the present invention seeks toprovide, in a semiconductor device, a termination circuit for providingon-die termination for a terminal of the semiconductor device that isconnected to an internal portion of the semiconductor device, whereinthe termination circuit comprises a MOS transistor connected between theterminal and a power supply; and control circuitry for driving a gate ofthe MOS transistor with a gate voltage, the control circuitry beingconfigured to control the gate voltage so as to place the MOS transistorin an ohmic region of operation when on-die termination is enabled, thegate voltage being controllable within a range of voltages so as tocause the MOS transistor when in the ohmic region of operation to imparta desired resistance within a range of resistances corresponding to therange of voltages.

In an embodiment, the MOS transistor is an NMOS transistor and the powersupply supplies a voltage that is less than lowest voltage within saidrange of voltages.

In an embodiment, the MOS transistor is a PMOS transistor and the powersupply supplies a voltage that is greater than the greatest voltagewithin said range of voltages.

In an embodiment, the power supply supplies a first voltage, thetermination circuit further comprises at least one circuit elementbetween the terminal and a second power supply that supplies a secondvoltage different from the first voltage. Also, in an embodiment, the atleast one circuit element comprises a resistive device, the MOStransistor is a first MOS transistor, and the at least one circuitelement comprises a second MOS transistor complementary to the first MOStransistor.

In an embodiment, the MOS transistor is a first MOS transistor, thetermination circuit further comprises a plurality of MOS transistorsconnected between the terminal and the power supply, and the pluralityof MOS transistors includes the first MOS transistor.

In an embodiment, the termination circuit is implemented on a firstsemiconductor chip, and the power supply is implemented on a secondsemiconductor chip different from the first semiconductor chip.

In an embodiment, the termination circuit and the power supply areimplemented on the same semiconductor chip.

In an embodiment, the termination circuit further comprises the powersupply, the power supply comprises a bias stage, an output stage and acapacitor, the output stage including a complementary pair of MOStransistors, the voltage supplied by the power supply is taken from ajunction between the complementary pair of MOS transistors, and thecapacitor is electrically connected between the junction and a referencepotential.

In an embodiment, the termination circuit further comprises the powersupply, the power supply comprises (i) a bias chain; (ii) an operationalamplifier in a unity-gain configuration having an input connected to thebias chain and an output; and (iii) a capacitor connected between theoutput of the operational amplifier and a reference potential, and thevoltage supplied by the power supply is taken from a junction betweenthe output of the operational amplifier and the capacitor.

In an embodiment, the MOS transistor comprises a gate and a pair ofcurrent carrying electrodes, one of the current carrying electrodes isconnected to the terminal, and the other of the current carryingelectrodes is connected to the power supply and the gate is driven bythe gate voltage from the control circuitry.

In an embodiment, the MOS transistor further comprises a substrateelectrode connected to a power supply that supplies a substrate voltage.

In an embodiment, to place the MOS transistor in an ohmic region ofoperation, the gate voltage is set to a first voltage, and the substratevoltage is different from the first voltage.

In an embodiment, the MOS transistor is an NMOS transistor, the firstvoltage is approximately 1.8 V, and the substrate voltage is atapproximately 0 V.

In an embodiment, the MOS transistor is a PMOS transistor, the firstvoltage is approximately 0 V, and the substrate voltage is atapproximately 1.8 V.

In an embodiment, the first voltage is taken from a cell substrateback-bias power supply.

In an embodiment, the first voltage is taken from a wordline powersupply.

In an embodiment, the control circuitry is further configured to placethe plurality of transistors in an off state when on-die termination isdisabled. In an embodiment, the control circuitry comprises an input forreceiving an enable signal indicative of whether on-die termination isenabled or disabled.

In an embodiment, the control circuitry comprises calibrator circuitryconfigured to carry out a calibration process for identifying an analogcalibration voltage that would cause the MOS transistor to impart thedesired resistance if it were supplied to the MOS transistor as the gatevoltage. In an embodiment, the calibration circuitry carries out thecalibration process in response to detecting that a received calibrationenable signal has been asserted.

In an embodiment, the control circuitry further comprises a multiplexerfor causing the analog calibration voltage to be transferred to the gatevoltage when on-die termination is enabled.

In an embodiment, the calibrator circuitry comprises an internal circuitelement exhibiting a behaviour as a function of an applied voltage thatcorresponds to a behavior of the MOS transistor as a function of thegate voltage, and the calibration process comprises determining theanalog calibration voltage as the level of the applied voltage thatresults in the internal circuit element exhibiting a resistance thatsubstantially equals the desired resistance.

In an embodiment, the calibrator circuitry has access to a lookup tablespecifying a resistance behavior of the MOS transistor as a function ofthe gate voltage, and the calibration process comprises consulting thelookup table to determined the analog calibration voltage on a basis ofthe desired resistance.

In an embodiment, the electrical resistance between the power supply andthe terminal is attributable in substantial part to the MOS transistor.

In an embodiment, the semiconductor device has a second terminalconnected to the internal portion, the termination circuit furthercomprises a second MOS transistor connected between the second terminaland the power supply. In this embodiment, the control circuitry isfurther for driving a gate of the second MOS transistor with a secondgate voltage, the control circuitry is further configured to control thesecond gate voltage so as to place the second MOS transistor in an ohmicregion of operation when on-die termination is enabled, and the secondgate voltage is controllable within a second range of voltages so as tocause the second MOS transistor when in the ohmic region of operation toimpart a second desired resistance within a second range of resistancescorresponding to the second range of voltages.

In an embodiment, the MOS transistor and the second MOS transistor areboth NMOS transistors or are both PMOS transistors, and the range ofvoltages is the second range of voltages.

In an embodiment, the MOS transistor and the second MOS transistor arecomplementary MOS transistors, and the range of voltages is differentfrom the second range of voltages.

According to a fifth broad aspect, the present invention seeks toprovide a semiconductor device with on-die termination, which comprisesan internal portion; a power supply; a terminal connected to theinternal portion; a MOS transistor connected between the terminal andthe power supply; control circuitry for driving a gate of the MOStransistor with a gate voltage, the control circuitry being configuredto control the gate voltage so as to place the MOS transistor in anohmic region of operation when on-die termination is enabled, the gatevoltage being controllable within a range of voltages so as to cause theMOS transistor while in the ohmic region of operation to impart adesired resistance within a range of resistances corresponding to therange of voltages.

According to a sixth broad aspect, the present invention seeks toprovide a semiconductor device with on-die termination, which comprisesan internal portion; a power terminal for connection to an off-chippower supply; a data terminal connected to the internal portion; a MOStransistor connected between the data terminal and the power terminal;and control circuitry for driving a gate of the MOS transistor with agate voltage, the control circuitry being configured to control the gatevoltage so as to place the MOS transistor in an ohmic region ofoperation when on-die termination is enabled, the gate voltage beingcontrollable within a range of voltages so as to cause the MOStransistor while in the ohmic region of operation to impart a desiredresistance within a range of resistances corresponding to the range ofvoltages.

These and other aspects and features of the present invention will nowbecome apparent to those of ordinary skill in the art upon review of thefollowing description of specific embodiments of the invention inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1 and 2 are circuit diagrams of a termination circuit forproviding on-die termination for a terminal of a semiconductor device,in accordance with specific non-limiting embodiments of the presentinvention;

FIG. 3A is a block diagram of a termination control circuit equippedwith digital calibration functionality, for use with the terminationcircuit of FIGS. 1 and 2;

FIG. 3B is a block diagram of a termination control circuit equippedwith analog calibration functionality, for use with the terminationcircuit of FIGS. 1 and 2;

FIG. 3C is a circuit diagram of a multiplexer that can be used in thetermination control circuit of FIG. 3B;

FIGS. 4A and 4B are circuit diagrams of a voltage generator forgenerating a voltage that can be supplied to the termination circuit ofFIGS. 1 and 2;

FIG. 5 is a circuit diagram of a termination circuit for providingon-die termination for a plurality of terminals of a semiconductordevice, in accordance with a specific non-limiting embodiment of thepresent invention;

FIGS. 6A and 6B are circuit diagrams showing complementary versions of alevel shifter that can be used to expand the range of a voltage signal,in accordance with specific non-limiting embodiments of the presentinvention; and

FIGS. 7 and 8 are circuit diagrams of a termination circuit forproviding on-die termination for a terminal of a semiconductor device,in accordance with other specific non-limiting embodiments of thepresent invention.

It is to be expressly understood that the description and drawings areonly for the purpose of illustration of certain embodiments of theinvention and are an aid for understanding. They are not intended to bea definition of the limits of the invention.

DETAILED DESCRIPTION

With reference to FIGS. 1 and 2, there is shown a termination circuit500 for on-die termination of a terminal 14 connected to an internalportion 16 of a semiconductor device 100, 200. On-die termination can beused to preserve the integrity of a signal that is transmitted and/orreceived via the terminal 14. Accordingly, the terminal 14 can be aninput terminal, an output terminal or a bidirectional input/outputterminal. In certain non-limiting embodiments, the terminal 14 can beconfigured to transmit and/or receive data signals varying between twovoltage levels representative of corresponding logic values. Thesemiconductor device 100, 200 that includes the internal portion 16 andthe terminal 14 may be a memory chip (such as a dynamic random accessmemory (DRAM), synchronous DRAM (SDRAM), double data rate (DDR) SDRAM,etc.) or any other type of semiconductor device that can benefit fromon-die termination.

Although the termination circuit 500 is shown as being connected withinsemiconductor device 100, 200 to a point (or node) 18 that is betweenthe terminal 14 and the internal portion 16 of the semiconductor device100, 200, it should be appreciated that it is within the scope ofembodiments of the present invention for the termination circuit 500 tobe connected directly to the terminal 14. Internal portion 16 mayinclude input buffers, output buffers, combined input/output buffers,memory peripheral circuits, memory arrays (composed of DRAM, NAND Flash,NOR Flash, or other types of memory cells), to name a few non-limitingpossibilities. The termination circuit 500 also includes a path betweennode 18 and a power supply 450, which is at a voltage V_(TT).

As shown in FIG. 1, power supply 450 can be internal to thesemiconductor device 100, in which case V_(TT) can be said to begenerated in an on-chip fashion. Alternatively, as shown in FIG. 2,power supply 450 can be external to the semiconductor device 200 andaccessible via a terminal 210, for example. In this case, V_(TT) can besaid to be generated in an off-chip fashion. Power supply 450 can alsobe used for supplying the voltage V_(TT) to other components of thesemiconductor device 100, 200, such as those comprised in the internalportion 16. Alternatively, power supply 450 can be dedicated to the taskof on-die termination.

The path between terminal 14 and power supply 450 (via point/node 18)includes a plurality of metal oxide semiconductor (MOS) transistors. Atleast one of the MOS transistors is a PMOS transistor and at least oneof the MOS transistors is an NMOS transistor. In the illustratedembodiment, there are four (4) MOS transistors 502, 504, 506, 508, amongwhich MOS transistors 502 and 504 are PMOS transistors and MOStransistors 506 and 508 are NMOS transistors. It should be appreciated,however, that there is no particular limitation on the number of MOStransistors in the path or on whether a particular MOS transistor in thepath is a PMOS transistor or an NMOS transistor, except for the factthat there will be at least two MOS transistors including at least onePMOS transistor and at least one NMOS transistor. Also, the path betweenterminal 14 and power supply 450 (via point/node 18) can include MOStransistors placed in parallel, in series or a combination thereof.

Each of the MOS transistors 502, 504, 506, 508 includes a respectivegate 502G, 504G, 506G, 508G, which those skilled in the art willunderstand to be a control electrode. The gate 502G, 504G, 506G, 508G ofeach of the MOS transistors 502, 504, 506, 508 is driven by a respectivegate voltage EN_502, EN_504, EN_506, EN_508 supplied by terminationcontrol circuit 528A, 528B.

In addition, each of the MOS transistors 502, 504, 506, 508 includes arespective first current carrying electrode 502S, 504S, 506S, 508S and arespective second current carrying electrode 502D, 504D, 506D, 508D. Oneof the current carrying electrodes of each of the MOS transistors 502,504, 506, 508 is connected to power supply 450, while the other of thecurrent carrying electrodes of each of the MOS transistors 502, 504,506, 508 is connected to terminal 14 (via point/node 18). Depending onwhich current carrying electrode is at a higher potential, either thefirst current carrying electrode will act as the “source” and the secondcurrent carrying electrode will act as the “drain”, or vice versa.

Furthermore, each of the MOS transistors 502, 504, 506, 508 includes arespective substrate electrode 502T, 504T, 506T, 508T. The substrateelectrode 502T, 504T of each of the PMOS transistors 502, 504 isconnected to a power supply 410 via a pin 110, while the respectivesubstrate electrode 506T, 508T of each of the NMOS transistors 506, 508is connected to a power supply 420 via a pin 120. Power supply 410 canbe maintained at a voltage V_(DD), while power supply 420 can be kept ata voltage V_(SS). The voltages V_(DD) and V_(SS) can be selected suchthat they provide sufficient voltage “headroom” to allow the componentsof the semiconductor device 100, 200 and, in particular, the terminationcircuit 500, to function properly within the expected voltage swing ofthe signals at the terminal 14. Thus, when the signals at the terminal14 are expected to vary between, say, 0.45V and 1.35V, it is possible toset V_(DD) to 1.8V and to set V_(SS) to 0V. If terminal 14 is an outputterminal, the voltages V_(DD) and V_(SS) may also be employed to powerthe output buffer. In DDR SDRAM these voltages are referred to asV_(DDQ) and V_(SSQ). Other possibilities are contemplated as beingwithin the scope of certain embodiments of the present invention, e.g.,V_(DD) could be set to 1.5V.

The termination control circuit 528A, 528B receives an “ODT enable”signal (denoted ODT_EN) which indicates enabling or disabling of on-dietermination. The termination control circuit 528A, 528B is configured torespond to assertion of the ODT_EN signal by causing all or less thanall of the gate voltages EN_502, EN_504, EN_506, EN_508 to change, thusprovoking a change in conductive state of the corresponding one(s) ofthe MOS transistors 502, 504, 506, 508.

More specifically, when the ODT_EN signal is de-asserted (i.e., whenon-die termination is disabled), the termination control circuit 528A,528B is configured so as to cause the gate voltages EN_502 and EN_504 tobe sufficiently high (e.g., V_(DD)) so as to ensure that PMOStransistors 502 and 504 are placed in the off state and to cause thegate voltages EN_506 and EN_508 to be sufficiently low (e.g., V_(SS)) soas to ensure that NMOS transistors 506 and 508 are placed in the offstate. In the off state, each of the MOS transistors 502, 504, 506, 508effectively acts as an open circuit between the respective first currentcarrying electrode 502S, 504S, 506S, 508S and the respective secondcurrent carrying electrode 502D, 504D, 506D, 508D.

In contrast, when the ODT_EN signal is asserted (i.e., when on-dietermination is enabled), the termination control circuit 528A, 528Bcauses some (or all) of the gate voltages EN_502, EN_504, EN_506, EN_508to change so as to acquire a level suitable for placing thecorresponding MOS transistor in the “ohmic region of operation”. By“ohmic region of operation”, which can also be referred to as “linearregion” or “triode region”, is meant a conductive state of a MOStransistor wherein a substantially linear relationship exists betweenthe drain-source voltage drop and the current flowing through thecurrent carrying electrodes (drain and source). Persons skilled in theart will understand that by “substantially linear relationship” one doesnot require the relationship to be perfectly linear, only that it bemore linear than when the MOS transistor is in either an off state orsaturation.

The level of the gate voltage suitable for placing a particular MOStransistor in the ohmic region of operation is a function of, amongpossibly other parameters: (i) whether the particular MOS transistor isan NMOS transistor or a PMOS transistor; (ii) the voltage V_(TT) ofpower supply 450; and (iii) the threshold voltage of the particular MOStransistor. One can define operation in the ohmic region as taking placewhen the drain-source voltage drop is less than the gate-source voltagedrop minus the threshold voltage. However, this is only one possibledefinition.

From the above, it will be apparent that the conductive state in whichthe MOS transistors 502, 504, 506, 508 find themselves at a given pointin time may be influenced by the instantaneous voltage at the terminal14. In particular, for a given MOS transistor operating in the ohmicregion of operation, the voltage at the terminal 14 may, during peaks orvalleys, occasionally push the given MOS transistor out of the ohmicregion and into a different region of operation. This does notconstitute an impermissible situation. Overall, it should be appreciatedthat the level of the gate voltage suitable for placing the given MOStransistor in the ohmic region of operation can be a level that ensuresoperation in the ohmic region of operation throughout a substantialrange of the expected voltage swing of the signal at the terminal 14,and need not guarantee that operation in the ohmic region is maintainedcontinuously throughout the entire expected voltage swing of the signalat the terminal 14.

Thus, for example, when V_(TT)=0.9V and the voltage at the terminal 14is expected to swing between 0.45V and 1.35V, a specific non-limitingexample of a gate voltage that places one of the PMOS transistors 502,504 in the ohmic region of operation is V_(SS)=0V (which is also thevoltage of power supply 420 that supplies the substrate electrodes 506T,508T). When the transistor in question is one of the NMOS transistors506, 508, it can be placed in the ohmic region of operation by settingthe gate voltage to V_(DD)=1.8V (which is also the voltage of powersupply 410 that is supplied to the substrate electrodes 502T, 504T).With such an arrangement, the PMOS and NMOS transistors now operate inthe ohmic region of operation throughout a substantial range of theexpected voltage swing of the signal at the terminal 14.

It is noted that V_(TT), which was described earlier as being thevoltage level of power supply 450, is greater than the gate voltage thatplaces the PMOS transistors 502, 504 in the ohmic region of operationand less than the gate voltage that places the NMOS transistors 506, 508in the ohmic region of operation. In a specific non-limiting embodiment,V_(TT) can be substantially midway between the two voltages V_(SS) andV_(DD), e.g., V_(TT)=0.9 when V_(SS)=0 V and V_(DD)=1.8 V. However, thisis only one possibility. For example, in an embodiment to be describedlater with reference to FIGS. 6A and 6B, a PMOS transistor can be placedin the ohmic region of operation by a gate voltage lower than V_(SS),and an NMOS transistor can be placed in the ohmic region of operation bya gate voltage higher than V_(DD). In such a case, V_(TT) is againintermediate the two voltages, and possibly midway, between althoughthis is not a requirement.

It should be appreciated that by using a single power supply at V_(TT)that connects to a current carrying electrode of each of the PMOStransistors 502, 504 and the NMOS transistors 506, 508, the terminationcircuit 500 consumes less power than a split termination designemploying two power supplies at V_(SS) and V_(DD).

It should also be appreciated that a given one of the MOS transistors502, 504, 506, 508 that is placed in the ohmic region of operationeffectively acts as a resistor with a resistance that is approximated bythe quotient of the drain-source voltage drop and the current flowingthrough the current carrying electrodes (drain and source). It is alsonoted that the path between power supply 450 and terminal 14 (viapoint/node 18) can be kept free of passive resistors. As such, it willbe apparent that conductivity between terminal 14 and power supply 450(via point/node 18) is attributable in substantial part to those MOStransistors that are placed in the ohmic region of operation (since theMOS transistors in the off state act as open circuits). Additionally, itwill be apparent that the electric resistance between terminal 14 andpower supply 450 (via point/node 18) is attributable in substantial partto the MOS transistors 502, 504, 506, 508 collectively, regardless ofwhether they are in an off state (in which case they act as an opencircuit) or is placed in the ohmic region of operation (in which casethey act as resistors).

It should further be appreciated that placing different subsets of theMOS transistors 502, 504, 506, 508 in the ohmic region of operationallows different electrical resistances to be imparted to the pathbetween terminal 14 and power supply 450. In particular, the terminationcontrol circuit 528A, 528B can be used to control the electricalresistance of the path by placing some of the MOS transistors 502, 504,506, 508 in the ohmic region of operation, while keeping the remainingMOS transistors in the off state. Exactly which subset of the MOStransistors 502, 504, 506, 508 should be placed in the ohmic region ofoperation can be determined by way of a calibration process, as will nowbe described.

Specifically, with reference to FIG. 3A, in a non-limiting embodiment,the calibration process is digital. That is to say, each of the gatevoltages EN_502, EN_504, EN_506, EN_508 provided by termination controlcircuit 528A varies between a respective first voltage at which thecorresponding one of the MOS transistors 502, 504, 506, 508 is placed inthe off state, and a respective second voltage at which thecorresponding one of the MOS transistors 502, 504, 506, 508 is placed inthe ohmic region of operation.

Termination control circuit 528A provides digital calibrationfunctionality using calibration circuit 302A, latch 304 and enablecircuit 305A. Calibration circuit 302A is connected to latch 304, whichis in turn connected to enable circuit 305A. A reference resistor 306 isshown as being accessed by the calibration circuit 302A through a pindenoted Z_(Q), although it should be understood that in someembodiments, the reference resistor 306 may be internal to thecalibration circuit 302A or may even be omitted. The reference resistor306 represents the desired termination resistance to be achieved by thetermination circuit 500, and is a design parameter. Alternatively, thereference resistor 306 may represent a multiple or fraction of thedesired termination resistance to be achieved by the termination circuit500, and the calibrated ODT resistance will be scaled accordingly. Thecalibration circuit 302A receives a “calibration enable” (CAL_EN) signalfrom a controller (not shown) which can be asserted to indicate a desireof such controller to carry out a calibration process using thecalibrator circuitry 302A. Specifically, responsive to assertion of theCAL_EN signal, the calibrator circuitry 302A attempts to find a subsetof the MOS transistors 502, 504, 506, 508 which, when placed in theohmic region of operation, imparts a resistance (from the perspective ofterminal 14) that best approximates the resistance of the referenceresistor 306.

To this end, the calibration circuit 302A may comprise internalresistive devices (e.g., replica resistors) that are designed to havethe same resistance as the MOS transistors 502, 504, 506, 508 when theseare placed in the ohmic region of operation. The calibration circuit302A identifies a subset of the internal replica resistors whosecollective resistance matches that of the reference resistor 306. Thiscan be in an iterative fashion, starting with an initial subset of theinternal replica resistors and ending with a final, selected subset ofthe internal replica resistors.

In an alternative embodiment, the calibration circuit 302A includes orotherwise has access to a look-up table (not shown) that stores dataregarding the resistance values of the various MOS transistors 502, 504,506, 508, were they to be placed in the ohmic region of operation. Insuch an embodiment, the calibration circuit 302A obtains the resistanceof the reference resistor 306 (either by receiving a value from anexternal source or by measuring it directly), and then identifies asubset of resistance values (i.e., a subset of individual MOStransistors) that results in a satisfactory numerical match with respectto the resistance of the reference resistor 306.

Other ways of achieving resistance matching will become apparent tothose of skill in the art.

It should be appreciated that the subset of MOS transistors ultimatelyidentified includes at least one NMOS transistor and at least one PMOStransistor, and may include up to and including all of the MOStransistors between node 18 and power supply 450.

The calibration circuit 302A provides latch 304 with a plurality ofdigital calibration signals 382, 384, 386, 388, respectivelycorresponding to the MOS transistors 502, 504, 506, 508. The digitalcalibration signal corresponding to a particular MOS transistor will beat a voltage level that depends on (i) whether the particular MOStransistor is an NMOS or a PMOS device, and (ii) whether the particularMOS transistor is to be placed in the ohmic region of operation, asdetermined by the calibration circuit 302A. For example, the digitalcalibration signal for a PMOS transistor that is to be placed in the offstate can be set to V_(DD), the digital calibration signal for a PMOStransistor that is to be placed in the ohmic region of operation can beset to V_(SS), the digital calibration signal for an NMOS transistorthat is to be placed in the off state can be set to V_(SS), and thedigital calibration signal for an NMOS transistor that is to be placedin the ohmic region of operation can be set to V_(DD).

Latch 304 latches the values of the digital calibration signals 382,384, 386, 388 received from the calibration circuit 302A and transfersthem to enable circuit 305A in the form of latched digital calibrationsignals 392, 394, 396, 398. The latching operation of latch 304 can betriggered by de-assertion of the CAL_EN signal. The latched digitalcalibration signals 392, 394, 396, 398 will retain the same voltagelevels until the CAL_EN signal is asserted and then de-asserted again,for example, during a subsequent iteration of the calibration process.Thus, use of the latch 304 allows the calibration circuit 302A to bedisabled until needed again, thus the calibration circuit 302A does notunnecessarily dissipate current when it is not being used. Rather, thelevels of the latched digital calibration signals 392, 394, 396, 398 areretained by latch 304, which is simple to implement and has low powerconsumption.

Within enable circuit 305A, each of the latched digital calibrationsignals 392, 394, 396, 398 is received and logically combined (forexample, using a combination of logic AND and logic OR gates) with theODT_EN signal to yield a corresponding one of the gate voltages EN_502,EN_504, EN_506, EN_508. Specifically, when the ODT_EN signal goes highto indicate that on-die termination is enabled, the latched digitalcalibration signals 392, 394, 396, 398 are transferred unchanged throughthe enable circuit 305A to gate voltages EN_502, EN_504, EN_506, EN_508.Thus, where the latched digital calibration signal corresponding to aparticular one of the MOS transistors is at a level suitable for placingthat MOS transistor in the off state, the gate voltage destined for thatMOS transistor will acquire this same level. Similarly, where thelatched digital calibration signal corresponding to a particular one ofthe MOS transistors is at a level suitable for placing that MOStransistor in the ohmic region of operation, the gate voltage destinedfor that MOS transistor will acquire this same level.

On the other hand, when the ODT_EN signal goes low to indicate thaton-die termination is disabled, all of the gate voltages EN_502, EN_504,EN_506, EN_508 are forced to a level suitable for placing thecorresponding MOS transistors in the off state, namely V_(SS) (in thecase of an NMOS transistor) or V_(DD) (in the case of a PMOStransistor). Stated differently, the level of any of the latched digitalcalibration signals 392, 394, 396, 398 received from the calibrationcircuit 302A is overridden by disabling on-die termination.

It should be appreciated that the subset of MOS transistors placed inthe ohmic region through action of the termination control circuit 528Awhen on-die termination is enabled includes at least one NMOS transistorand at least one PMOS transistor, and may include up to and includingall of the MOS transistors between node 18 and power supply 450.

With reference now to FIG. 3B, in another non-limiting embodiment, thecalibration process is analog. That is to say, each of the gate voltagesEN_502, EN_504, EN_506, EN_508 provided by termination control circuit528B varies between a respective first voltage at which thecorresponding one of the MOS transistors 502, 504, 506, 508 is placed inthe off state, and a respective range of second voltages within whichthe gate voltages EN_502, EN_504, EN_506, EN_508 can vary stepwise orcontinuously to as to provide a fine-tuned resistance. Specifically,when a given one of the gate voltages EN_502, EN_504, EN_506, EN_508 isin the respective range of second voltages, the corresponding one of theMOS transistors 502, 504, 506, 508 is placed in the ohmic region ofoperation and imparts a variable resistance that depends on the value ofthe given one of the gate voltages EN_502, EN_504, EN_506, EN_508. Thus,the resistance of each of the MOS transistors 502, 504, 506, 508 can becontrolled to a certain degree of precision.

Termination control circuit 528B provides analog calibrationfunctionality using calibration circuit 302B. The aforementionedreference resistor 306 is shown as being accessed by the calibrationcircuit 302B through the aforementioned pin denoted Z_(Q), although itshould be understood that in some embodiments, the reference resistor306 may be internal to the calibration circuit 302B or may even beomitted. The reference resistor 306 represents the desired terminationresistance to be achieved by the termination circuit 500, and is adesign parameter. The calibration circuit 302B receives theaforementioned CAL_EN signal from a controller (not shown) which can beasserted to indicate a desire of such controller to carry out acalibration process using the calibration circuit 302B. Specifically,responsive to assertion of the CAL_EN signal, the calibration circuit302B attempts to find a subset of the MOS transistors 502, 504, 506, 508which, when placed in the ohmic region of operation, can be made tocollectively impart a resistance (from the perspective of node 18) thatbest approximates the resistance of the reference resistor 306.

To this end, the calibration circuit 302B may comprise calibrationcircuit elements that have the same resistance behavior as a function ofan applied voltage as the MOS transistors 502, 504, 506, 508 have as afunction of the gate voltages EN_502, EN_504, EN_506, EN_508,respectively. The calibration circuit 302B identifies which appliedvoltages, when applied to the calibration circuit elements, yield acollective resistance that matches the resistance of the referenceresistor 306. This can be done in an iterative fashion, starting with aninitial subset of applied voltages and ending with a final subset ofapplied voltages. The applied voltages in the final subset are output tomultiplexer 305B in the form of analog calibration voltages 372, 374,376, 378, respectively corresponding to the MOS transistors 502, 504,506, 508.

In an alternative embodiment, the calibration circuit 302B includes orotherwise has access to a look-up table (not shown) that stores dataregarding the resistance behaviour as a function of gate voltage of thevarious MOS transistors 502, 504, 506, 508, particularly in the ohmicregion of operation. In such an embodiment, the calibration circuit 302Bprovides processing functionality. Specifically, once the calibrationcircuit 302B obtains the resistance of the reference resistor 306(either by receiving a value from an external source or by measuring itdirectly), the calibration circuit 302B consults the look-up table todetermine the gate voltage that should be applied to each of the MOStransistors 502, 504, 506, 508 so as to achieve a satisfactory matchwith respect to the resistance of the reference resistor 306. The gatevoltages so determined are output to the multiplexer 305B in the form ofthe analog calibration voltages 372, 374, 376, 378.

Other ways of achieving resistance matching will become apparent tothose of skill in the art.

It should be appreciated that the analog calibration voltagecorresponding to a particular MOS transistor among the MOS transistors502, 504, 506, 508 will be at a voltage level that depends on (i)whether the particular MOS transistor is an NMOS or a PMOS device, (ii)whether the particular MOS transistor is to be placed in the ohmicregion of operation and (iii) assuming the particular MOS transistor isindeed to be placed in the ohmic region of operation, the preciseresistance sought to be imparted by the particular MOS transistor. Forexample, the analog calibration voltage for a PMOS transistor that is tobe placed in the off state can be set to V_(DD), the analog calibrationvoltage for a PMOS transistor that is to be placed in the ohmic regionof operation can be set within a range bounded by V_(S1) and V_(S2)(which may or may not include V_(SS)), the analog calibration voltagefor an NMOS transistor that is to be placed in the off state can be setto V_(SS), and the analog calibration voltage for an NMOS transistorthat is to be placed in the ohmic region of operation can be set towithin a range bounded by V_(D1) and V_(D2) (which may or may notinclude V_(DD)).

The analog calibration voltages 372, 374, 376, 378 are selectivelyswitched depending on the state of the ODT_EN signal within multiplexer305B to yield a corresponding one of the gate voltages EN_502, EN_504,EN_506, EN_508. Specifically, when the ODT_EN signal goes high toindicate that on-die termination is enabled, the analog calibrationvoltages 372, 374, 376, 378 are transferred unchanged through themultiplexer 305B to gate voltages EN_502, EN_504, EN_506, EN_508. Thus,where the analog calibration voltage corresponding to a particular oneof the MOS transistors 502, 504, 506, 508 is at a level suitable forplacing that MOS transistor in the off state, the gate voltage destinedfor that MOS transistor will acquire this same level. Similarly, wherethe analog calibration voltage corresponding to a particular one of theMOS transistors 502, 504, 506, 508 is at a level suitable for placingthat MOS transistor in the ohmic region of operation so as to impart acertain desired resistance, the gate voltage destined for that MOStransistor will acquire this same level.

On the other hand, when the ODT_EN signal goes low to indicate thaton-die termination is disabled, all of the gate voltages EN_502, EN_504,EN_506, EN_508 are forced to a level suitable for placing thecorresponding MOS transistors in the off state, namely V_(SS) (in thecase of an NMOS transistor) or V_(DD) (in the case of a PMOStransistor). Stated differently, the level of any of the analogcalibration voltages 372, 374, 376, 378 received from calibrationcircuit 302B is overridden by disabling on-die termination. It should beappreciated that calibration circuit 302B and multiplexer 305B need notbe separate and indeed may be combined into a single module.

As a non-limiting example, multiplexer 305B may be implemented with CMOStransmission gates comprised of pairs of parallel NMOS and PMOStransistors as shown in FIG. 3C. For the case where analog calibrationvoltages 372, 374, 376, 378 range between V_(SS) and V_(DD), the PMOStransistor substrates (not shown) can be tied to V_(DD), the NMOStransistor substrates (not shown) can be tied to V_(SS), and theinverter can be powered by V_(SS) and V_(DD). When the ODT_EN signal islow, the output of the inverter will be high, and the transmission gatesconnected between analog calibration voltages 372, 374, 376, 378 andgate voltages EN_502, EN_504, EN_506, EN_508 will be off, since the NMOStransistor in each transmission gate will have a low gate voltage, andthe PMOS transistor in each transmission gate will have a high gatevoltage. At the same time the transmission gates connected between fixedV_(SS) and V_(DD) levels and gate voltages EN_502, EN_504, EN_506,EN_508 will be on, since the NMOS transistor in each transmission gatewill have a high gate voltage, and the PMOS transistor in eachtransmission gate will have a low gate voltage. High gate voltagesEN_502, EN_504 disable PMOS termination transistors 502, 504. Low gatevoltages EN_506, EN_508 disable NMOS termination transistors 506, 508.

When the ODT_EN signal is high, the output of the inverter will be low,and the transmission gates connected between analog calibration voltages372, 374, 376, 378 and gate voltages EN_502, EN_504, EN_506, EN_508 willbe turned on, since the NMOS transistor in each transmission gate willhave a high gate voltage, and the PMOS transistor in each transmissiongate will have a low gate voltage. At the same time the transmissiongates connected between fixed V_(SS) and V_(DD) levels and gate voltagesEN_502, EN_504, EN_506, EN_508 will be off, since the NMOS transistor ineach transmission gate will have a low gate voltage, and the PMOStransistor in each transmission gate will have a high gate voltage.Analog calibration voltages 372, 374, 376, 378 are provided totermination transistors 502, 504, 506, 508 to enable on-die termination.

It should be appreciated that the subset of MOS transistors placed inthe ohmic region through action of the termination control circuit 528Bwhen on-die termination is enabled includes at least one MOS transistor,either a single PMOS transistor or a single NMOS transistor, and mayinclude up to and including all of the MOS transistors between node 18and power supply 450. Although a single transistor or a plurality oftransistors of a single type, either NMOS or PMOS, may be provided, itis possible to provide a plurality of transistors including at least oneNMOS transistor and at least one PMOS transistor. As the voltage onterminal 14 varies between a high and low voltage, an NMOS transistormay fall out of linear operation towards one end of the range while aPMOS transistor will fall out of linear operation towards the other endof the range. If NMOS and PMOS transistors are provided and calibratedto have similar or equal resistances at the midpoint of the range ofvoltages on terminal 14, the non-linearity effects at either of theextremes of the range can reduced.

It should also be appreciated that in some embodiments, a hybridanalog/digital approach can be used, with the effect that certain onesof the gate voltages EN_502, EN_504, EN_506, EN_508 may be derived fromdigital calibration signals and certain other ones of the gate voltagesEN_502, EN_504, EN_506, EN_508 may be derived from analog calibrationsignals.

Reference is now made to FIGS. 4A and 4B, which show example on-chipvoltage generators 600A, 600B for generating the voltage V_(TT) fromavailable voltage supplies at V_(DD) and V_(SS), in the specificnon-limiting example where V_(SS)=0V (ground) and V_(TT)=½V_(DD). InFIG. 4A, voltage generator 600A includes a bias stage 602 and an outputstage 604. Bias stage 602 includes a PMOS device 606 with its gate wiredto ground and an NMOS device 608 with its gate wired to V_(DD). Betweenthe two devices are connected a further PMOS device 610 and a furtherNMOS device 612. PMOS device 610 has its gate wired to junction 609situated between its source and the drain of NMOS device 608, while NMOSdevice 612 has its gate wired to junction 611 situated between its drainand the source of PMOS device 606. The output stage 604 includes an NMOSdevice 614 and a PMOS device 616 connected in series between V_(DD) andground. A V_(TT) node 620 is located at junction 613 situated betweenNMOS device 614 and PMOS device 616, while an output capacitance 618shunts the V_(TT) node 620 to ground.

The illustrated voltage generator 600A has the benefit that currentthrough the bias stage 602 and the output stage 604 is relatively lowwhile V_(TT) is at the desired ½V_(DD) level. PMOS device 606 with itsgate wired to ground and NMOS device 608 with its gate wired to V_(DD)serve as resistors to limit the current within the bias stage 602.Moreover, the output stage 604 draws relatively little current whileV_(TT) is at the desired ½V_(DD) level because NMOS device 614 and PMOSdevice 616 each have a gate-source bias of approximately V_(T), namelythe threshold voltage. Once the output at the V_(TT) node 620 moves awayfrom the desired ½V_(DD) level, the gate-source bias of one of theoutput devices 614, 616 increases to provide a larger current to restorethe output level to ½V_(DD). The output capacitance 618 is provided as areservoir and can be made sufficiently large to supply instantaneouscurrent demands on the V_(TT) node 620. Optionally, voltage generator600A may share a common bias stage with other voltage sources on thesemiconductor device, which for a memory chip could include a source atV_(CP) (cell plate voltage) and/or a source at V_(BLP) (bitlineprecharge voltage).

In voltage generator 600B of FIG. 4B, a bias chain 650 (implemented as aresistor divider) sets a node 652 at a reference level. The voltage atthe node 652 is buffered by an operational amplifier 654 in a unity gainconfiguration. A V_(TT) node 656 is located at the output of theoperational amplifier 654, and is shunted to ground by an outputcapacitance 658. In some embodiments, the operational amplifier 654 hasa class B or class AB output stage where quiescent current is muchsmaller than the active current that flows to its output when V_(TT)diverges from the desired reference level. In addition to providing thedominant pole for closed loop stability, the output capacitance 658 canbe made sufficiently large to supply instantaneous current demands onthe V_(TT) node 656. In other words, the output capacitance 658 allowsthe circuitry 600B to supply sufficient current to maintain the V_(TT)node 656 at the proper level (in this case, V_(TT)=½V_(DD)) even in theworst case scenario when all terminals (such as the terminal 14) arecontinuously receiving ‘0’s or are continuously receiving ‘1’s. Thus, aseparate compensation capacitor internal to the operational amplifier654 is not required. For the in-between scenarios when some inputs arereceiving ‘1’ and others are receiving ‘0’, the input currents willactually cancel out at the V_(TT) node 656 and the current driverequirements of the operational amplifier 654 will be lower.

It should be appreciated that the above embodiments, which have beendescribed in the context of a single terminal 14, are also applicable inthe context of multiple terminals, be they input terminals, outputterminals, input/output terminals or a combination thereof. Inparticular, and with reference to FIG. 5, there is shown a schematicdiagram of a semiconductor device 700 in accordance with another exampleembodiment. The illustrated semiconductor device 700 has an 8-bitdatabus with 8 data terminals 714 ₀ . . . 714 ₇ connected to inputbuffers leading to an internal portion 716. Those skilled in the artwill appreciate that the databus can be bidirectional; however forsimplicity output buffers are not shown in FIG. 5.

Semiconductor device 700 comprises a termination circuit 500M connectedbetween the plurality of data terminals 714 ₀ . . . 714 ₇ and theinternal portion 716 of the semiconductor device 700. Terminationcircuit 500M includes a plurality of NMOS termination transistors 704Nand a plurality of PMOS termination transistors 704P. NMOS terminationtransistors 704N and PMOS termination transistors 704P each include asource and a drain, one of which is connected to the junction betweenthe internal portion 716 and a corresponding one of the data terminals714 ₀ . . . 714 ₇. The other of the source and the drain is connected toa common pin 702 which supplies the aforementioned voltage V_(TT) foron-die termination. In other embodiments, the voltage V_(TT) may begenerated on-chip as previously described with reference to FIGS. 4A and4B, for example.

Termination circuit 500M comprises control circuit 728, which disablesand enables on-die termination functionality based on an ODT_EN signal.The ODT_EN signal can be provided to control circuit 728 via a pin 730of the semiconductor device 700. In a non-limiting example, on-dietermination may be enabled when the semiconductor 700 is in receivingmode but disabled when the semiconductor device 700 is driving theterminals 714.

Based on the level of the ODT_EN signal, control circuit 728 sets thelevel of a gate voltage EN_704N fed to the gate of each of the NMOStermination transistors 704N and the level of a gate voltage EN_704P fedto the gate of each of the PMOS termination transistors 704P.Specifically, when the ODT_EN signal is de-asserted, control circuit 728causes the gate voltage EN_704N to take on a level that ensures that theNMOS termination transistors 704N are placed in the off state, anexample of such a level being V_(SS). Control circuit 728 also causesthe gate voltage EN_704P to take on a level that ensures that the PMOStermination transistors 704P are placed in the off state, an example ofsuch a level being V_(DD).

In contrast, when the ODT_EN signal is asserted, control circuit 728causes the gate voltage EN_704N to take on a level that ensures that theNMOS termination transistors 704N are placed in the ohmic region ofoperation. In some embodiments, an example of such a level is a fixedvoltage such as V_(DD). In other embodiments, an example of such a levelvaries within a range bound by V_(D1) and V_(D2), allowing the NMOStermination transistors 704N to impart a variable resistance. Controlcircuit 728 also causes the gate voltage EN_704P to take on a level thatensures that the PMOS termination transistors 704P are placed in theohmic region of operation. In some embodiments, an example of such alevel is a fixed voltage such as V_(SS). In other embodiments, anexample of such a level varies within a range bound by V_(S1) andV_(S2), allowing the PMOS termination transistors 704P to impart avariable resistance.

It should be appreciated that in the aforementioned example, bothtermination transistors connected to each data terminal were placed inthe ohmic region of operation when on-die termination was enabled.However, it should be appreciated that in some embodiments, there may bemultiple mixed PMOS and NMOS termination transistors connected to one ormore data terminals, in which case it may be desirable to identify whichsubset of these termination transistors should be placed in the ohmicregion of operation so as to achieve a desired termination resistancevalue.

It should be appreciated that in each of the above embodiments, the sizeof the MOS transistors can be reduced while still imparting the desiredresistance. In particular, it is remarked that when a MOS transistor isplaced in the ohmic region of operation, the current through the drain(denoted I_(D)) is approximately related to the drain-source voltagedrop (denoted V_(DS)) and the gate-source voltage drop (i.e., the gatevoltage, denoted V_(GS)) by the following equation (see page 310 ofMicroelectronic Circuits, Third Edition, by Adel S. Sedra and Kenneth C.Smith, Saunders College Publishing, 1991, hereby incorporated byreference herein):I _(D)=2K(V _(GS) −V _(T))V _(DS),

where V_(T) is the threshold voltage of the MOS transistor in questionand K is a device parameter given by:K=½μ_(η) C _(OX)(W/L),

where μ_(η) is the “electron mobility”, C_(OX) is the “oxidecapacitance”, L is the channel length of the MOS transistor and W is thechannel width of the MOS transistor. Thus, the resistance imparted bythe MOS transistor, which is expressed as R_(MOS)=V_(DS)/I_(D), equalsto:R _(MOS) =V _(DS) /I _(D)=(2K(V _(GS) −V _(T)))⁻¹ =L/(μ_(η) ·C _(OX)·W·(V _(GS) −V _(T))).

Thus, R_(MOS) is inversely proportional to both the channel width W andthe gate voltage V_(GS). It follows that while keeping the samegate-source voltage V_(GS), it is possible to achieve a largerresistance by a smaller MOS transistor. Conversely, a desired resistancecan be achieved using a smaller MOS transistor by supplying a greatergate-source voltage V_(GS). By “smaller” MOS transistor, it iscontemplated that the channel width W may be shrunk, while the channellength L is kept constant for ESD (Electro-Static Discharge) protectionconsiderations. However, this is only one example way in which to reducethe size of a MOS transistor.

Thus, the trade-off for using smaller MOS transistors for providing adesired resistance when in the ohmic region of operation is the need tosupply a stronger voltage at the gate. For an NMOS transistor, thistranslates into supplying a gate voltage greater than V_(DD) (while thesubstrate electrode is at V_(SS)) and for a PMOS transistor, thistranslates into supplying a gate voltage less than V_(SS) (while thesubstrate electrode is at V_(DD)).

In some embodiments, a dedicated power supply can be provided forgenerating these stronger gate voltages. However, in other embodiments,existing power supplies that are already at the stronger voltages can bere-used. This is the case with certain memory modules comprising anarray of memory cells accessed through wordlines and bitlines. In such acase, an example of a voltage above V_(DD) that may be re-used is theV_(PP) power supply that is otherwise employed for activating wordlinesin a DRAM, and an example of a voltage below power supply that may bere-used is the V_(BB) supply that is otherwise employed for cellsubstrate back-bias in a DRAM. Other possibilities exist and are withinthe scope of embodiments of the present invention.

Having established the desirability, in some circumstances, of supplyinggate voltages with a dynamic range that exceeds that which existsbetween V_(SS) and V_(DD), there are various ways of achieving this. Forexample, from a power conservation point of view, it may be desirable toproceed with a two-step process, whereby the gate voltages are firstgenerated as previously described in the case of termination controlcircuit 528A (namely, with a dynamic range of V_(SS) to V_(DD)), andthen the dynamic range of the gate voltages is augmented using levelshifters. Specifically, level shifters such as the one shown at 802 inFIG. 6A can be inserted in the paths between termination control circuit528A and the gates of PMOS transistors 502, 504 in FIGS. 1 and 2.Similarly, level shifters such as the one shown at 852 in FIG. 6B can beinserted in the paths between termination control circuit 528A and thegates of NMOS transistors 506, 508. It should be appreciated that thelevel shifters can be inserted in the paths between termination controlcircuit 528A and all of the transistors 502, 504, 506, 508 or only asubset of the transistors 502, 504, 506, 508. Thus, it is possible thattransistors of the same type (e.g., NMOS or PMOS) are provided withdifferent gate voltages that place those transistors in the ohmic regionof operation.

In the example embodiment shown in FIG. 6A, level shifter 802 convertsan input voltage EN_502 (which is assumed to be a binary signal having alevel that is either V_(SS) or V_(DD)) into a level shifted outputvoltage EN_502+ (which will be a binary signal having a level that iseither V_(BB) or V_(DD)). Here, V_(BB) represents a voltage level thatis lower than V_(SS). In a non-limiting example, V_(SS) may be 0V andV_(BB) may be −1.0V. Other possibilities exist and are contemplated asbeing within the scope of certain embodiments of the present invention.

Specifically, level shifter 802 comprises two interconnected branches ofMOS transistors 804, 806. The first branch 804 comprises PMOS transistor808 whose gate receives input voltage EN_502. The source of PMOStransistor 808 is connected to power supply V_(DD) and the drain of PMOStransistor 808 is connected to the drain of NMOS transistor 810. Thesource of NMOS transistor 810 is connected to power supply 812 at avoltage V_(BB)<V_(SS). The second branch 806 comprises PMOS transistor814 whose source is also connected to V_(DD) and whose drain isconnected to the drain of NMOS transistor 816. The source of NMOStransistor 816 is connected to power supply 812 at voltage V_(BB). Thegate of PMOS transistor 814 is connected to the output of inverter 811which inverts the input voltage EN_502. Also, the gate of NMOStransistor 810 in the first branch 804 is connected to the drain of NMOStransistor 816 in the second branch 806. In addition, the gate of NMOStransistor 816 in the second branch 806 is connected to the drain ofNMOS transistor 810 in the first branch 804. Finally, the level shiftedoutput voltage EN_502+ is taken at node 820 between the drain of PMOStransistor 814 and the drain of NMOS transistor 816. Those skilled inthe art will thus appreciate from FIG. 6A that when input voltage EN_502is at V_(SS), the level shifted output voltage EN_502+ is at V_(BB), andwhen input voltage EN_502 is at V_(DD), the level shifted output voltageEN_502+ is at V_(DD).

In the example embodiment shown in FIG. 6B, level shifter 852 convertsan input voltage EN_506 (which is assumed to be a binary signal having alevel that is either V_(SS) or V_(DD)) into a level shifted outputvoltage EN_506+ (which will be a binary signal having a level that iseither V_(SS) or V_(PP)). Here, V_(PP) represents a voltage level thatis higher than V_(DD). In a non-limiting example, V_(DD) may be 1.8V andV_(PP) may be 2.5V. Other possibilities exist and are contemplated asbeing within the scope of certain embodiments of the present invention.

Specifically, level shifter 852 comprises two interconnected branches ofMOS transistors 854, 856. The first branch 854 comprises an NMOStransistor 858 whose gate receives input voltage EN_506. The source ofNMOS transistor 858 is connected to power supply V_(SS) and the drain ofNMOS transistor 858 is connected to the drain of PMOS transistor 860.The source of PMOS transistor 860 is connected to power supply 862 at avoltage V_(PP)>V_(DD). The second branch 856 comprises NMOS transistor864 whose source is connected to power supply V_(SS) and whose drain isconnected to the drain of a PMOS transistor 866. The source of PMOStransistor 866 is connected to power supply 862 at V_(PP). The gate ofNMOS transistor 864 is connected to the output of inverter 861 whichinverts the input voltage EN_506. Also, the gate of PMOS transistor 860in the first branch 854 is connected to the drain of PMOS transistor 866in the second branch 856. In addition, the gate of PMOS transistor 866in the second branch 856 is connected to the drain of PMOS transistor860 in the first branch 854. Finally, the level shifted output voltageEN_506+ is taken at node 870 between the drain of NMOS transistor 864and the drain of PMOS transistor 866. Those skilled in the art will thusappreciate from FIG. 6B that when input voltage EN_506 is at V_(SS), thelevel shifted output voltage EN_506+ is at V_(SS), and when inputvoltage EN_506 is at V_(DD), the level shifted output voltage EN_506+ isat V_(PP).

It should be appreciated that the symbols “V₀₀”, “V_(SS)”, “V_(PP)” and“V_(BB)”, which may seem familiar to some readers, are used for merelyillustrative purposes as an aid to placing the voltage levels of variouspower supplies in context relative to one another. However, the actualvoltage levels represented by the symbols “V_(DD)”, “V_(SS)”, “V_(PP)”and “V_(BB)” are not constrained to only those specific voltage levelsthat the reader may have come across by consulting the literature, norare they prohibited from acquiring voltage levels that the reader mayhave come across as being represented in the literature by differentsymbols or by no symbol at all.

It should also be appreciated that analog termination control circuit528B described above with reference to FIG. 3B can be used in animplementation of a semiconductor device having exclusively NMOStransistors or exclusively PMOS transistors, and as few as a single MOStransistor of one type or the other. Also, analog termination controlcircuit 528B can be used in an implementation of a semiconductor deviceirrespective of the voltage level provided by the V_(TT) terminationvoltage power supply 450. Accordingly, reference is made to FIG. 7,where there is shown a termination circuit 901 for on-die termination ofa terminal 914 connected to the internal portion 916 of a semiconductordevice 900. The terminal 914 can be an input terminal, an outputterminal or a bidirectional input/output terminal. In certainnon-limiting embodiments, the terminal 914 can be configured to transmitand/or receive data signals varying between two voltage levelsrepresentative of corresponding logic values. The semiconductor device900 that includes the internal portion 916 and the terminal 914 may be amemory chip or any other type of semiconductor device that can benefitfrom on-die termination.

Although the termination circuit 901 is shown as being connected withinsemiconductor device 900 to a point (or node 918) that is between theterminal 914 and the internal portion 916 of the semiconductor device900, it should be appreciated that it is within the scope of embodimentsof the present invention for the termination circuit 901 to be connecteddirectly to the terminal 914. The termination circuit 901 includes apath between terminal 914 and a power supply 950 via the point/node 918,which is at a voltage V_(XYZ). The voltage V_(XYZ) can be a mid-pointtermination voltage such as V_(DD)/2, a pseudo open-drain terminationvoltage such as V_(DD), a near ground termination voltage such asV_(SS), or any other suitable termination voltage. As shown in FIG. 7,power supply 950 can be internal to the semiconductor device 900, inwhich case V_(XYZ) can be said to be generated in an on-chip fashion.Alternatively, power supply 950 can be external to the semiconductordevice 900 and accessible via a data terminal, for example. In thiscase, V_(XYZ) can be said to be generated in an off-chip fashion. Powersupply 950 can also be used for supplying the voltage V_(XYZ) to othercomponents of the semiconductor device 900, such as those comprised inthe internal portion 916. Alternatively, power supply 950 can bededicated to the task of on-die termination.

The path between terminal 914 and power supply 950 (via the point/node918) includes at least one MOS transistor, including MOS transistor 902.The at least one MOS transistor, including MOS transistor 902, can be aPMOS transistor or an NMOS transistor. In the illustrated embodiment,there is one (1) MOS transistor 902, shown as an NMOS transistor, but itshould be appreciated that there is no particular limitation on thenumber of MOS transistors in the path or on whether a particular MOStransistor in the path is a PMOS transistor or an NMOS transistor. Also,the path between terminal 914 and power supply 950 (via the point/node918) can include MOS transistors placed in parallel, in series or acombination thereof.

MOS transistor 902 includes a gate 902G, which those skilled in the artwill understand to be a control electrode. The gate 902G is driven by agate voltage EN_902 supplied by termination control circuit 928.

In addition, MOS transistor 902 includes a first current carryingelectrode 902S and a second current carrying electrode 902D. One of thecurrent carrying electrodes is connected to power supply 950, while theother of the current carrying electrodes is connected to terminal 914(via the point/node 918). Depending on which current carrying electrodeis at a higher potential, either the first current carrying electrodewill act as the “source” and the second current carrying electrode willact as the “drain”, or vice versa.

Furthermore, MOS transistor 902 includes a substrate electrode 902T. Thesubstrate electrode 902T is connected to power supply 910 via a pin (notshown). For an NMOS transistor 902 as shown, power supply 910 can bemaintained at a voltage V_(SS). The voltage V_(SS) can be selected suchthat it provides sufficient voltage “headroom” to allow the componentsof the semiconductor device 900 and, in particular, the terminationcircuit 901, to function properly within the expected voltage swing ofthe signals at the terminal 914. Thus, when the signals at the terminal914 are expected to vary between, say, 0.0V and 0.6V, it is possible toset V_(SS) to 0V. Other possibilities are contemplated as being withinthe scope of certain embodiments of the present invention.

Termination control circuit 928 is configured to respond to assertion ofan ODT_EN signal by causing the gate voltage EN_902 to change, thusprovoking a change in conductive state of MOS transistor 902. Morespecifically, when the ODT_EN signal is de-asserted (i.e., when on-dietermination is disabled), the termination control circuit 928 isconfigured so as to cause the gate voltage EN_902 to be sufficiently low(e.g., V_(SS)) so as to ensure that an NMOS transistor 902 is placed inthe off state. In the off state, MOS transistor 902 effectively acts asan open circuit between the first current carrying electrode 902S andthe respective second current carrying electrode 902D.

In contrast, when the ODT_EN signal is asserted (i.e., when on-dietermination is enabled), termination control circuit 928 causes the gatevoltage EN_902 to change so as to acquire a level suitable for placingMOS transistor 902 in the ohmic region of operation.

The level of the gate voltage suitable for placing MOS transistor 902 inthe ohmic region of operation is a function of, among possibly otherparameters: (i) the fact that MOS transistor 902 is an NMOS transistor;(ii) the voltage V_(XYZ) of power supply 950; and (iii) the thresholdvoltage of MOS transistor 902. From the above, it will be apparent thatthe conductive state in which MOS transistor 902 finds itself at a givenpoint in time may be influenced by the instantaneous voltage at theterminal 914. In particular, the voltage at terminal 914 may, duringpeaks or valleys, occasionally push MOS transistor 902 out of the ohmicregion and into a different region of operation. This does notconstitute an impermissible situation. Overall, it should be appreciatedthat the level of the gate voltage suitable for placing MOS transistor902 in the ohmic region of operation can be a level that ensuresoperation in the ohmic region of operation throughout a substantialrange of the expected voltage swing of the signal at terminal 914, andneed not guarantee that operation in the ohmic region is maintainedcontinuously throughout the entire expected voltage swing of the signalat terminal 914.

Thus, for example, when V_(XYZ)=V_(SS)=0V and the voltage at terminal914 is expected to swing between 0V and 0.6V, a specific non-limitingexample of a gate voltage range that places MOS transistor 902 in theohmic region of operation (for a typical transistor threshold voltageV_(T) of 0.5V) is 0.9V to 1.2V. With such an arrangement, MOS transistor902 now operates in the ohmic region of operation throughout asubstantial range of the expected voltage swing of the signal atterminal 914 while allowing analog control of the terminationresistance.

It is noted that V_(XYZ), which was described earlier as being thevoltage level of power supply 950, is less than the gate voltage thatplaces MOS transistor 902 in the ohmic region of operation. The oppositewould be true if MOS transistor 902 were a PMOS transistor.

In a specific non-limiting embodiment, V_(XYZ) can be substantiallymidway between the two voltages V_(SS) and V_(DD), e.g., V_(XYZ)=0.9when V_(SS)=0 V and V_(DD)=1.8 V. However, this is only one possibility.Other possibilities include a split termination scenario, as shown inFIG. 8, which illustrates a termination circuit 1001 similar to thetermination circuit 901 of FIG. 7 but where V_(XYZ) is set to V_(DD),while an additional MOS transistor 902* complementary to MOS transistor902 is provided between node 918 and V_(DD). MOS transistor 902* is aPMOS transistor while MOS transistor 902 continues to be an NMOStransistor.

It should be appreciated that when MOS transistors 902 and 902* areplaced in the ohmic region of operation, they effectively act as aresistors with a resistance that is approximated by the quotient of thedrain-source voltage drop and the current flowing through the currentcarrying electrodes (drain and source). It is also noted that the pathbetween power supply 950 and node 918 and the path between power supply910 and node 918 can be kept free of passive resistors. As such, it willbe apparent that conductivity between node 918 and power supplies 950and 910 is attributable in substantial part to MOS transistors 902 and902* having been placed in the ohmic region of operation. Additionally,it will be apparent that the electric resistance between node 918 andpower supplies 950, 910 is attributable in substantial part to MOStransistors 902 and 902*, regardless of whether they are in an off state(in which case they act as an open circuit) or are placed in the ohmicregion of operation (in which case they act as a resistor).

It should further be appreciated that varying the gate voltages EN_902and EN_902* allows different electrical resistances to be imparted tothe path between node 918 and power supplies 950 and 910. In particular,a slightly modified termination control circuit 928* can be used tocontrol the electrical resistance of the path by controlling the gatevoltages EN_902 and EN_902*. Specifically, the gate voltage EN_902provided by the termination control circuit 928* varies between a firstvoltage at which MOS transistor 902 is placed in the off state, and arange of second voltages within which the gate voltage EN_902 can varystepwise or continuously, while the gate voltage EN_902* provided by thetermination control circuit 928* varies between a first voltage at whichMOS transistor 902* is placed in the off state, and a range of secondvoltages within which the gate voltage EN_902* can vary stepwise orcontinuously. Specifically, when the gate voltages EN_902 and EN_902*are in the range of second voltages, MOS transistors 902 and 902* areplaced in the ohmic region of operation and impart variable resistancesthat depend on the value of the gate voltages EN_902 and EN_902*,respectively. Thus, the resistances of MOS transistors 902 and 902* canbe controlled to a certain degree of precision.

The termination control circuit 928* provides analog calibrationfunctionality using calibration circuit 952 and multiplexer 955. Areference resistor (not shown) can be being accessed by the calibrationcircuit 952 through an external pin of the semiconductor device 900,although it should be understood that in some embodiments, the referenceresistor may be internal to the calibration circuit 952 or may even beomitted. The reference resistor represents the desired terminationresistance to be achieved by the termination circuit 950, and is adesign parameter. The calibration circuit 952 receives a “calibrationenable” (CAL_EN) signal from a controller (not shown) which can beasserted to indicate a desire of such controller to carry out acalibration process using the calibration circuit 952.

In one embodiment, the calibration circuit 952 may comprise acalibration circuit element (or multiple calibration circuit elements)that has (or have) the same resistance behaviour as a function of anapplied voltage as MOS transistor 902 and/or 902* have as a function ofthe gate voltage EN_902 and/or EN_902*. Thus, responsive to assertion ofthe CAL_EN signal, the calibration circuit 952 identifies which appliedvoltage(s), when applied to the calibration circuit element(s), yield(s)a resistance that matches the resistance of the reference resistor. Thiscan be done in an iterative fashion, starting with an initial appliedvoltage and ending with a final applied voltage. The final appliedvoltages are output to the multiplexer 955 in the form of analogcalibration voltages 972 and/or 976.

In an alternative embodiment, the calibration circuit 952 includes orotherwise has access to a look-up table (not shown) that stores dataregarding the resistance behaviour of MOS transistors 902 and/or 902* asa function of gate voltage, particularly in the ohmic region ofoperation. In such an embodiment, the calibration circuit 952 providesprocessing functionality. Specifically, since the calibration circuit952 obtains the resistance of the reference resistor (either byreceiving a value from an external source or by measuring it directly),the calibration circuit 952 consults the look-up table to determine thegate voltage that should be applied to the MOST transistors 902 and/or902* so as to achieve a satisfactory match with respect to theresistance of the reference resistor. The gate voltages so determinedare output to the multiplexer 955 in the form of the analog calibrationvoltages 972 and/or 976.

Other ways of achieving resistance matching will become apparent tothose of skill in the art.

It should be appreciated that the analog calibration voltage 972 will beat a voltage level that takes into consideration the fact that MOStransistor 902 is an NMOS device and depends on whether MOS transistor902 is to be placed in the ohmic region of operation and, if so, theprecise resistance sought to be imparted by MOS transistor 902. Forexample, the analog calibration voltage can be set to V_(SS) when MOStransistor 902 is to be placed in the off state, and can be set towithin a range bounded by V_(D1) and V_(D2) (which may or may notinclude V_(DD)) when MOS transistor 902 is to be placed in the ohmicregion of operation.

It should also be appreciated that the analog calibration voltage 976will be at a voltage level that takes into consideration the fact thatMOS transistor 902* is a PMOS device and depends on whether MOStransistor 902* is to be placed in the ohmic region of operation and, ifso, the precise resistance sought to be imparted by MOS transistor 902*.For example, the analog calibration voltage can be set to V_(DD) whenMOS transistor 902* is to be placed in the off state, and can be set towithin a range bounded by V_(S1) and V_(S2) (which may or may notinclude V_(SS)) when MOS transistor 902* is to be placed in the ohmicregion of operation.

For a split termination implementation, both NMOS and PMOS devices areusually both enabled or both disabled. When enabled, calibrating theresistances of the NMOS and PMOS devices to be equal results in aneffective termination voltage at the mid-point between V_(DD) and V_(SS)and an effective termination resistance equal to one half of thecalibrated resistance value of either of the NMOS or PMOS devices.

The analog calibration voltages are selected by the ODT_EN signal at themultiplexer 955 to yield gate voltage EN_902 and EN_902*. Specifically,when the ODT_EN signal goes high to indicate that on-die termination isenabled, the analog calibration voltages are transferred unchangedthrough the multiplexer 955 to gate voltages EN_902 and EN_902*. Thus,where the analog calibration voltages are at levels suitable for placingMOS transistors 902 and 902* in the off state, the gate voltages EN_902and EN_902* will acquire these levels. Similarly, where the analogcalibration voltages are at levels suitable for placing MOS transistors902 and 902* in the ohmic region of operation so as to impart certaindesired resistances, the gate voltages EN_902 and EN_902* will acquirethese levels.

On the other hand, when the ODT_EN signal goes low to indicate thaton-die termination is disabled, the gate voltages EN_902 and EN_902* areforced to levels suitable for placing MOS transistors 902 and 902* inthe off state, namely V_(SS) and V_(DD), respectively. Stateddifferently, the levels of the analog calibration voltages received fromthe calibration circuit 952 are overridden by disabling on-dietermination. It should be appreciated that the calibration circuit 952and the multiplexer 955 need not be separate and indeed may be combinedinto a single module.

In the context of the examples described above, various elements andcircuits are shown connected to each other for the sake of simplicity.In practical applications of the present invention, elements, circuits,etc. may be connected directly to each other. As well, elements,circuits etc. may be connected indirectly to each other through otherelements, circuits, etc., necessary for operation of the devices,systems or apparatus of which they form a part. Thus, in actualconfiguration, the various elements and circuits can be directly orindirectly coupled with or connected to each other, unless otherwisespecified.

Certain adaptations and modifications of the described embodiments canbe made. Therefore, the above discussed embodiments are to be consideredillustrative and not restrictive. Also it should be appreciated thatadditional elements that may be needed for operation of certainembodiments of the present invention have not been described orillustrated as they are assumed to be within the purview of the personof ordinary skill in the art. Moreover, certain embodiments of thepresent invention may be free of, may lack and/or may function withoutany element that is not specifically disclosed herein.

What is claimed is:
 1. A termination circuit for a terminal of asemiconductor device, the terminal associated with an expected voltageswing, the termination circuit comprising: a plurality of NMOStransistors directly connected to the terminal and connected to a powersupply at a supply voltage; a plurality of PMOS transistors directlyconnected to the terminal and connected to the power supply; and controlcircuitry for controllably enabling and disabling on-die termination,wherein when on-die termination is enabled, the control circuitry isconfigured to drive at least one of the NMOS transistors with acorresponding NMOS gate voltage that is greater than the supply voltageand greater than the maximum voltage of the expected voltage swing, andis further configured to drive at least one of the PMOS transistors witha corresponding PMOS gate voltage that is less than the supply voltageand less than the minimum voltage of the expected voltage swing.
 2. Thetermination circuit defined in claim 1, wherein the corresponding NMOSgate voltage for each of the at least one of the NMOS transistors is acommon first voltage, and wherein the corresponding PMOS gate voltagefor each of the at least one of the PMOS transistors is a common secondvoltage.
 3. The termination circuit defined in claim 2, wherein thesupply voltage is substantially midway between the common first voltageand the common second voltage.
 4. The termination circuit defined inclaim 1, wherein the corresponding NMOS gate voltages for at least twoof the NMOS transistors are different.
 5. The termination circuitdefined in claim 1, wherein the termination circuit provides anelectrical resistance between the power supply and the terminal, theelectrical resistance being attributable in substantial part to the atleast one of the NMOS transistors and the at least one of the PMOStransistors.
 6. The termination circuit defined in claim 1, wherein thecontrol circuitry is further configured to place the NMOS transistorsand the PMOS transistors in an off state when on-die termination isdisabled.
 7. The termination circuit defined in claim 6, wherein thecontrol circuitry comprises an input for receiving an enable signalindicative of whether on-die termination is enabled or disabled.
 8. Thetermination circuit defined in claim 1, wherein the control circuitrycomprises calibrator circuitry configured to carry out a calibrationprocess for selecting the at least one of the NMOS transistors and theat least one of the PMOS transistors from among the plurality of NMOStransistors and the plurality of PMOS transistors.
 9. The terminationcircuit defined in claim 8, wherein the calibrator circuitry comprises aplurality of internal resistive devices each matched to a resistanceassociated with resistive operation of a respective one of the NMOS andPMOS transistors, wherein the calibrator circuitry has access to areference resistance, wherein the calibration process comprisesdetermining a particular combination of the internal resistive deviceswhose collective resistance substantially equals the referenceresistance, wherein for each of the internal resistive devices in theparticular combination, the respective NMOS or PMOS transistor isselected as one of the at least one NMOS transistor or at least one PMOStransistor to be driven when on-die termination is enabled.
 10. Thetermination circuit defined in claim 8, wherein the calibrator circuitryhas access to a lookup table specifying a resistance associated withresistive operation of a respective one of the NMOS and PMOStransistors, wherein the calibration process comprises consulting thelookup table to identify a particular combination of the NMOS and PMOStransistors whose collective resistance substantially equals thereference resistance, wherein the NMOS and PMOS transistors in theparticular combination are selected as the at least one NMOS transistorand the at least one PMOS transistor to be driven when on-dietermination is enabled.
 11. The termination circuit defined in claim 1,wherein the control circuitry comprises calibrator circuitry with accessto a reference resistance, the calibrator circuitry configured to carryout a calibration process for selecting a plurality of analogcalibration voltages that would cause the at least one NMOS transistorand the at least one PMOS transistor to impart a resistancesubstantially equal to the reference resistance if supplied thereto asthe corresponding NMOS and PMOS gate voltages, respectively.
 12. Thetermination circuit defined in claim 11, wherein the control circuitryfurther comprises a multiplexer for causing the analog calibrationvoltages to be delivered as the corresponding NMOS and PMOS gatevoltages, respectively, when on-die termination is enabled.
 13. Thetermination circuit defined in claim 11, wherein the calibratorcircuitry comprises internal circuit elements each exhibiting a behavioras a function of an applied voltage that corresponds to a behavior ofone of the at least one of the NMOS transistors and the at least one ofthe PMOS transistors as a function of the corresponding NMOS or PMOSgate voltage, wherein the calibration process comprises selecting theanalog calibration voltages to be those levels of applied voltage thatresult in the internal circuit elements collectively exhibiting aresistance that substantially matches the reference resistance.
 14. Thetermination circuit defined in claim 11, wherein the calibratorcircuitry has access to a lookup table specifying a resistance behaviorof the at least one NMOS transistor and the at least one PMOS transistoras a function of the corresponding NMOS or PMOS gate voltage, whereinthe calibration process comprises consulting the lookup table on a basisof the reference resistance to determine particular voltages, theparticular voltages being the analog calibration voltages.
 15. Thetermination circuit defined in claim 1, the semiconductor device havinga second terminal connected to the internal portion, the terminationcircuit further comprising: a plurality of second NMOS transistorsconnected between the second terminal and the power supply; a pluralityof second PMOS transistors connected between the second terminal and thepower supply; and wherein when on-die termination is enabled, thecontrol circuitry is configured to drive at least one of the second NMOStransistors with a corresponding second NMOS gate voltage that isgreater than the supply voltage and greater than the maximum voltage ofthe expected voltage swing and is further configured to drive at leastone of the second PMOS transistors with a corresponding second PMOS gatevoltage that is less than the supply voltage and less than the minimumvoltage of the expected voltage swing.
 16. In combination, thetermination circuit defined in claim 1 and the power supply, wherein thetermination circuit is implemented on a first semiconductor chip andwherein the power supply is implemented on a second semiconductor chipdifferent from the first semiconductor chip.
 17. The termination circuitdefined in claim 1, wherein the termination circuit and the power supplyare implemented on the same semiconductor chip.
 18. A terminationcircuit for a terminal of a semiconductor device, the terminalassociated with an expected voltage swing, the termination circuitcomprising: a plurality of NMOS transistors, each comprising a gate, asubstrate electrode and a pair of current carrying electrodes, whereinone of the current carrying electrodes is directly connected to theterminal, wherein the other current carrying electrode is connected to apower supply at a supply voltage, and wherein the substrate electrode isconnected to a first substrate power supply that supplies a firstsubstrate voltage; a plurality of PMOS transistors, each comprising agate, a substrate electrode and a pair of current carrying electrodes,wherein one of the current carrying electrodes is directly connected tothe terminal, wherein the other current carrying electrode is connectedto the power supply at the supply voltage, and wherein the substrateelectrode is connected to a second substrate power supply that suppliesa second substrate voltage; control circuitry for controllably enablingand disabling on-die termination, wherein when on-die termination isenabled, the control circuitry is configured to drive at least one ofthe NMOS transistors with a corresponding NMOS gate voltage that isgreater than the second substrate voltage, and to drive at least one ofthe PMOS transistors with a corresponding PMOS gate voltage that is lessthan the first substrate voltage.
 19. The termination circuit defined inclaim 18, wherein the second voltage is taken from a cell substrateback-bias power supply.
 20. The termination circuit defined in claim 19,wherein the first voltage is taken from a wordline power supply.
 21. Thetermination circuit defined in claim 18, wherein the second substratevoltage is at approximately 1.8 V, and wherein the first substratevoltage is at approximately 0 V.
 22. The termination circuit defined inclaim 21, wherein the corresponding NMOS gate voltage for at least oneof the NMOS transistors is approximately 2.5 V and wherein thecorresponding PMOS gate voltage for at least one of the PMOS transistorsis approximately −1.0 V.
 23. The termination circuit defined in claim18, wherein at least two of the NMOS transistors or at least two of thePMOS transistors have the same channel width.
 24. The terminationcircuit defined in claim 18, wherein at least two of the NMOStransistors or at least two of the PMOS transistors have differentchannel widths.